Maximizing the power conversion efficiency of a tin perovskite/silicon thin-film tandem solar cell

ABSTRACT

A bi-facial tandem solar cell and a method of making a non-toxic perovskite/inorganic thin-film tandem solar cell stable, having matching bandgaps and a hysteresis free design including the steps of depositing a textured oxide buffer layer on an inexpensive substrate, depositing a metal-inorganic film from a eutectic alloy on the buffer layer; and depositing perovskite elements on the metal-inorganic film, thus forming a perovskite layer based on a metal from the metal-inorganic film, incorporating the metal into the perovskite layer wherein said perovskite layer is stable, hysteresis-free, and has a bandgap that matches the bandgap of the metal-inorganic.

This application is a Continuation in Part of U.S. patent applicationSer. No. 15/205,233, filed Jul. 8, 2016, entitled “TinPerovskite/Silicon Thin-Film Tandem Solar Cell,” which claims priorityto U.S. Provisional Patent Application No. 62/333,454 filed May 9, 2016,entitled “Tin Perovskite/Silicon Thin-Film Tandem Solar Cell.” Thepresent application also claims priority to U.S. Provisional Application62/334,745, filed May 11, 2016, entitled “Method of Stabilizing TinPerovskite/Silicon Thin-Films in a Tandem Solar Cell,” U.S. ProvisionalApplication 62/335,109, filed May 12, 2016, entitled “Method of Making aBandgap Compatible Tin Perovskite/Silicon Thin-Film Tandem Solar Cell,”and U.S. Provisional Application 62/336,826, filed May 16, 2016,entitled “Method of Making a Planar Hysteresis-Free Perovskite/SiliconThin-Film in a Tandem Solar Cell.” All of the applications referencedabove are hereby incorporated by reference their entirety.

The present invention relates to non-toxic perovskite/silicon thin-filmsolar cells, transistors, light emitting diodes, and other electronicdevices. More specifically the present invention relates to stability,compatible bandgaps in tandem designs, and hysteresis-free designs innon-toxic perovskite/silicon thin film solar cells.

BACKGROUND OF THE INVENTION

In the solar cell technology invented by the late Dr. Praveen Chaudhari,a method is disclosed (U.S. Pat. No. 9,054,249) for making a tandemsolar cell in which a “thin-silicon film 7934592.1 can be used forheteroepitaxial deposition of other semiconductors, which might be moreefficient converters of light to electricity.” The material“Perovskites,” although not new, has recently been the subject of atremendous amount of attention in the solar cell technology communitydue to the quick progress and achievement of high efficienciesdemonstrated with regard to light conversion for solar cell purposesover a relatively short period of time. The name ‘perovskite solar cell’is derived from the ABX₃ crystal structure of the absorber materials,which is referred to as perovskite structure. “Perovskites” is thenomenclature for any materials that adopt the same crystal structure ascalcium titanate (ABX₃). There are hundreds of different materials thatadopt this structure, with a multitude of properties, includinginsulating, antiferromagnetic, piezoelectric, thermoelectric,semiconducting, conducting, and, probably most famously,superconducting. (H. Snaith “Perovskites: The Emergence of a New Era forLow Cost, High Efficiency Solar Cells”, 2013). Nonetheless, thousands ofdifferent chemical compositions are possible as perovskites are a wideranging class of materials in which organic molecules made mostly ofcarbon and hydrogen bind with a metal, such as lead, and a halogen, suchas chlorine, in a three dimensional crystal lattice.

Many believe that solar cells will need to have a power conversionefficiency (PCE) around 25% and a cost below $0.5/W to revolutionize howthe world's population obtains its electricity. Perovskites' conversionefficiency has increased over the last five years from 4 percent tonearly 20 percent. The theoretical limit of perovskite's conversionefficiency is about 66 percent, compared to silicon's theoretical limitof about 32 percent. The ingredients used to create perovskite arewidely available and inexpensive to combine, since it can be done atrelatively low temperatures (around 100° C.). While there are manyadvantages to perovskites, there are also disadvantages. One of thecomponents of the perovskite commonly experimented with—MAPbI— is Pb orlead—a highly toxic metal. And while perovskite based solar cells havenot (yet) gained market entry, before they could do so any perovskitesolar cells would have to undergo extensive testing to make sure thatlead wouldn't be a risk factor. Although researchers have noted that theamount of lead present is relatively low, and would likely have a veryminimal negative environmental impact, a perovskite without a toxicmetal would be advantageous. Researchers have been able to producelead-free perovskite cells that swap lead out for tin, which couldeliminate the concern entirely. This tin (Sn) perovskite, when combinedwith another semiconductor material as a layer underneath for a tandemor multi junction structure, could lead to an ideal non-toxic solar cellcapable of solving current energy needs and to combating climate change.There is, however, another challenge. Since perovskite solar cellsalready have the efficiency that is needed for commercialization and canalmost certainly be manufactured at a highly attractive cost, theprimary barrier to commercialization is going to be obtaining long-termstability. The challenge appears to be that the films are highlyreactive with water and have a tendency to emit methylammonium iodide.This also holds for perovskite/silicon tandem solar cells. As of thedate of this disclosure, tandem solar cells with a lead based perovskiteand crystalline silicon bottom layer have been fabricated and reportedon. For example, methylammonium lead tri-halide perovskite and siliconsolar cells can form a complementary pair. With the perovskite solarcell functioning as a top layer, it can harvest the short wavelengthphotons while the bottom layer coated with silicon is designed to absorbthe long wavelength photons. As there are different wavelengths forsolar energy, a combination of different materials for making solarcells would work best for energy absorption.

In the fabrication of perovskite solar cells, it is well known thatthere are several challenges that must be overcome in order for thesolar cells made from these materials to become commercially viable.These challenges are: stability, compatible bandgaps in tandem designs,and hysteresis-free designs. The following invention solves theseissues.

If a tandem solar panel could reach 30 percent efficiency, the impact onthe balance-of-system cost could be enormous: only two thirds of thenumber of panels would be needed to produce the same amount of power aspanels that are 20 percent efficient, greatly reducing the amount ofroof space or land, installation materials, labor and equipment. Thechallenge is to produce good connections between semiconductors,something that has been challenging with regard to silicon because ofthe arrangement of silicon atoms in crystalline silicon The othermaterial(s), on the silicon sublayer, also presents challenges. And inthe case of perovskites film fabrication—how the film is made—is ofcrucial importance as it determines the film's texture, crystalstructure, composition, and defect formation that collectivelycontribute toward over-all device performance. Furthermore, interfaceengineering has proven to effectively optimize device performance as itaffects carrier dynamics across the entire device including chargegeneration, transportation, and collection. As will be seen, theinvention disclosed herein is directly related to all these issues.

The concept of the stacked solar cell was introduced to increase outputvoltage of a-Si:H solar cells. Only later it was recognized that stackedcells also offer a practical solution for improving the stabilizedperformance of a-Si:H based solar cells. Different terms such as tandemor dual junction or double junction solar cells are used in theliterature to describe a cell in which two junctions are stacked on topof each other. A stack of three junctions is named a triple junctionsolar cell. The multi junction solar cell structure is far more complexthan the single junction solar cell. For its successful operation thereare two crucial requirements: (i) the current generated at the maximumpower point has to be equal in each component cell (current matching)and (ii) an internal series connection between the component cells hasto feature low electrical and optical losses. The internal seriesconnection is accomplished at the p-n junction, where the recombinationof oppositely charged carriers arriving from the adjacent componentcells takes place.

The requirement of current matching reflects the fact that componentcells function as current sources which are connected in series. Thecomponent cell that generates the lowest current determines the netcurrent flowing through the stacked two terminal cell. In order to avoidcurrent losses, each component cell should generate the same current.The current generated by a component cell depends mainly on theabsorption in the absorber layer of the cell, which is determined by thethickness of the absorber. Current is matched by adjusting the thicknessof the absorber layer of each component cell.

The tunnel recombination junction deals with the interface between thecomponent cells. This interface is in fact a p-n diode. An ohmic contactbetween the component cells is required for proper operation of thestacked solar cell. The problem of obtaining the ohmic contact betweenthe component cells can be resolved by fabricating a so-called tunnelrecombination junction. This junction ensures that the electronsarriving at the n-type layer of the top cell and the holes arriving atthe p-type layer of the bottom cell fully recombine at this junction.The recombination of the photogenerated carriers at this interface keepsthe current flowing through the solar cell. A very high electric fieldin this reverse biased p-n junction facilitates tunneling of thecarriers towards the defect states in the center of the junction. Theeffective recombination of the carriers takes place through thesedefective states. A tunnel recombination junction is usually realized byusing microcrystalline silicon for at least one of the doped layers inorder to obtain good ohmic contact. Another approach is to incorporate athin oxide layer at the interface between the two component cells thatserves as an efficient recombination layer. When the p-n junctionfunctions as a good ohmic contact, the V_(oc) of the stacked cell is thesum of the open circuit voltages of the component cells.

SUMMARY OF THE INVENTION

As discussed in the present invention disclosed herein, the crystallinesilicon thin-film can serve as the tunneling junction, or the metal fromthe eutectic alloy can be oxidized to form the thin oxide layer at theinterface between the two component cells that serves as an efficientrecombination layer. Alternatively, the metal film can remainun-oxidized. Ohmic contacts to semiconductors are typically constructedby depositing thin metal films of a carefully chosen composition. Itshould be noted that in the technology invented by the late Dr. PraveenChaudhari (referred to in the introduction), no intermediate layerbetween the two semiconductor materials—say silicon and perovskites—isrequired.

Perovskite solar cells hold an advantage over traditional silicon solarcells in the simplicity of their processing. Traditional silicon cellsrequire expensive, multistep processes, conducted at high temperatures(>1000° C.) in a high vacuum in special clean room facilities. Meanwhilethe organic-inorganic perovskite material can be manufactured withsimpler wet chemistry techniques in a traditional lab environment. Mostnotably, methylammonium and formamidinium lead trihalides have beencreated using a variety of solvent techniques and vapor depositiontechniques, both of which have the potential to be scaled up withrelative feasibility. In vapor assisted techniques, spin coated orexfoliated lead halide is annealed in the presence of methylammoniumiodide vapor at a temperature of around 150° C. This technique holds anadvantage over solution processing, as it opens up the possibility formulti-stacked thin films over larger areas. Additionally, vapordeposited techniques result in less thickness variation than simplesolution processed layers. However, both techniques can provide thedesired result of planar thin film layers or for use in mesoscopicdesigns, such as coatings on a metal oxide scaffold.

As stated, ideally the perovskite material should be non-toxic and whencombined with silicon, a thin-film of silicon is preferable sincesilicon thin-films are less expensive and easier to fabricate than thecommonly used wafer (c-Si) In fact, in 2014 two teams independentlydeveloped perovskite cells that swap lead for tin. The chemical formulafor one of these perovskites was CH₃NH₃SnI₃ or MASnI or Methylammoniumtin triiodide (N. Noel et al “Lead-free organic-Inorganic tin halideperovskites for photovoltaic applications”). The following inventionfacilitates the formation of a non-toxic lead-free perovskite/siliconthin film solar cell by providing a way of forming a tin (or othernon-toxic metal such as Al, or Cu, etc. that forms a eutectic withsilicon or an inorganic material) based perovskite layer on a siliconthin-film which greatly simplifies the manufacturing of a non-toxicperovskite/silicon thin-film tandem solar cell.

For application in perovskite/silicon tandem cells, the perovskiteabsorber layer has to be highly transparent at photon energies below itsband gap. Any light absorbed in this sub-bandgap wavelength range wouldneither contribute to the photocurrent of the perovskite cell nor betransmitted to the silicon thin-film bottom cell, and therefore wouldseverely limit performance.

In fact, a tandem perovskite/silicon thin-film solar cell could haveefficiencies as high as 40 to 45% (this is the case if silicon is placedin tandem with any 1.65 to 1.7 eV band gap material, not justperovskites). It is advantageous to combine perovskite films that arenon-toxic with silicon thin-films. A method for fabricating thisstructure is disclosed herein and in a corresponding publication (A.Chaudhari “High Efficiency Perovskite/Crystalline Silicon Thin-FilmTandem Solar Cell from a Highly Textured MgO [111] Buffered GlassSubstrate: A Proposal”, submitted for publication in Journal of EnergyChallenges and Mechanics”, June 2016).

Also, highest efficiencies would be expected for perovskite band gaps of1.7-1.8 eV. Adding bromine into the perovskite material, the band gapcan be tuned to higher energies, thus making it more suitable for tandemapplications.

While perovskite single junction solar cells could become cheaper thansilicon wafers, which currently dominate the market, because they can bemade at much lower temperature, the tandem perovskite/silicon thin-filmallows for an even cheaper method because even when the perovskites areadded to a silicon wafer for a tandem cell, the silicon wafer stillneeds to be made at roughly ˜900 C in order to remove defects. Whereasthe method of growing inorganic crystalline thin-films on glass inventedby P. Chaudhari (U.S. Pat. No. 9,054,249) uses eutectics which allowsfor a drastic reduction in temperature, all the way down to 272° C., theeutectic temperature of tin-silicon (Sn—Si) alloy, for example. Thepresent invention makes use of the metal which forms on the inorganicthin-film on glass (or inexpensive substrate) to form the perovskite toplayer thus facilitating the formation of a tandem solar cell for highefficiency.

One of the challenges of solar cells is to produce good connectionsbetween semiconductors, something that has been challenging with regardto silicon because of the arrangement of silicon atoms in crystallinesilicon In this invention, the silicon (inorganic material) atoms have apreferred [111] orientation which is an advantage over polycrystallineor random orientation. The texture can also induce texture in theperovskite film formed on top, thereby improving the connection betweenthe semiconductors. In the case of monolithic tandems, the top cell isdirectly processed on the bottom cell. This has the advantage of areduced number of fabrication steps and fewer doped transparentconducting electrodes, resulting in lower manufacturing costs and lessparasitic absorption; however, monolithic tandems require strict processcompatibility, such that both top- and bottom-cell fabrication schemeshave to be specifically adapted for monolithic tandem integration: (i)both sub cells have to be optimized to produce the same current atmaximum power point, as the tandem current will be limited by the subcell with the lower current; (ii) the perovskite cell may have to beprocessed at low temperatures for temperature sensitive bottom cells,such as the amorphous/crystalline silicon heterojunction (SHJ) cell, thesilicon photovoltaic technology with currently the highest performance;and (iii) bottom cells with front surface texture are not compatiblewith solution processing, which is typically used for the deposition ofmany layers during perovskite cell fabrication.

The present invention provides a method for forming a non-toxic metalbased perovskite film on an inorganic thin-film.

The present invention also provides a method of forming a tin perovskitefilm on a silicon thin-film on glass.

The present invention also provides a facile method of forming a tandemperovskite/silicon thin film solar cell that is non-toxic.

The present invention also provides a method of forming a tandemperovskite/silicon thin film solar cell that is non-toxic in a simple,cost effective manner.

The present invention also improves the connection between the twosemiconductors in the tandem solar cell.

The present invention also induces a smooth and uniform, homogenoussemiconductor bottom layer onto which the top perovskite layer isdeposited.

The present invention provides a method for stabilizing makingcompatible bandgaps, and making hysteresis-free designs in tandemdesigns.

DETAILED DESCRIPTION OF THE INVENTION

In the current invention “perovskite” is an organic-inorganic metalhalide material, though it should be noted that the invention is notlimited to these hybrid perovskite compounds. For example, a non-organicperovskite could be used instead. The phenomenal performances of hybridperovskites stem from the substantial characteristic properties theypossess. Hybrid perovskites demonstrate a strong optical absorption, anadjustable band gap, long diffusion lengths, ambipolar charge transport,high carrier mobility, and a high tolerance of defects. The ability totune electronic and optical properties of hybrid perovskites with suchease presents a major attraction.

The thickness of the subcell is the critical factor for power conversionefficiencies (PCE) of tandem devices, and that the combination layer (orintermediate layer) shows good electrical connection in these tandemdevices.

It should also be noted that the invention disclosed here can be appliedto any semiconductor compound that has a metal that can form a eutecticalloy with silicon or other inorganic material such as germanium. Anexample of such a compound is AgBiS₂ where Ag (silver) forms a eutecticalloy with silicon.

A good high vacuum system with two electron beam guns, is used todeposit a metal such as Sn and an inorganic material such as siliconindependently. A glass substrate (or other inexpensive substrate) coatedwith a textured oxide such as MgO is held at temperatures between 575and 600° C. These are nominal temperatures. It is understood to oneskilled in the art that lower or higher temperatures can also be useddepending upon the softening temperature of the glass substrate or thereaction kinetics of either Sn or Si with the MgO layers when used assubstrates. A thin tin film of approximately 10 nm thickness isdeposited first. This is followed by a Si film deposited at a rate of 2nm per minute on top of the tin film. The silicon film nucleatesheterogeneously on the MgO surface to form the desired thin film. Thefilm can now be cooled to room temperature, where the film now comprisesof two phases: tin and a relatively large grained and highly texturedfilm of silicon on MgO. The tin diffuses to the surface of the siliconfilm, driven by its lower surface energy relative to the siliconsurface. Rather than etching the film in a solution, which removes theSn from the two phases, tin and silicon, leaving behind a silicon film(practiced in P. Chaudhari U.S. Pat. No. 9,054,249) the Sn in the Si—Snfilm can now be used as a surface on which to deposit the otherperovskite elements combining to form a tin perovskite. For the Si—Sndeposition we have used two electron beam guns as an illustrativeexample. It is understood to one skilled in the art that other methodssuch as a single gun with multiple hearths, chemical vapor deposition,thermal heating, or sputtering can also be used. For the perovskiteelements, such as methylammonium halides, deposition can be bylow-temperature solution methods (typically spin-coating, but alsoothers). Since low-temperature (below 100° C.) solution-processed filmstend to have considerably smaller diffusion lengths, a highertemperature method may however be favorable so long as it does not meltthe underlying substrate.

Since under ambient conditions the Sn2+ ion will rapidly oxidize to itsmore stable Sn4+ analogue, which destroys the charge neutrality of theperovskite structure and causes it to break down, preparation andsealing should ideally take place under inert atmosphere—i.e. undervacuum (N. Noel et al, “Lead-free organic-inorganic tin halideperovskites for photovoltaic applications”, 2014).

As already noted, a primary barrier to commercialization of perovskitesolar cells is long-term stability. This is certainly true for tinperovskite solar cells, where the Sn oxidizes from Sn2+ to Sn4+ and evenin inert atmosphere the perovskite—say CH₃NH₃SnI₃—becomes unstable.However, in the present invention stability of the tin perovskite filmis improved greatly by the fact that the tin layer formed on the siliconis already either crystallized, or crystallizing as the other elementsin the perovskite are added. This process lessens the reactivity of thematerial with water, for example, and improves overall film stability tothe point of overcoming the problem. Moreover, by doping the metal inthe perovskite, say Sn, with silicon or germanium, the stability can beimproved. Such doping can take place as a matter of course during theformation of the film, when the Sn diffuses to the surface of thesilicon or germanium film and the Sn picks up impurities along the way,or it can be introduced as a separate step.

Unlike Pb-based perovskite which requires heating to crystallize, the Snperovskites crystallize at room temperature. This is actually animpediment to uniform film formation (Noel et al). Therefore, having Snon the surface of the Si film in advance of the addition of the otherelements in the perovskite, can help achieve uniformity and smoothness.In other words, the Sn layer controls the crystallization of the tinperovskite. Moreover, since the Si layer is oriented, the perovskitelayer will also become oriented or textured.

The perovskite film can be deposited in the following way: Hybridperovskites can be prepared using different deposition routes and arecomprised of two main precursor components: an organic methylammoniumhalide cation (CH₃NH₃X, X═Cl, Br, I) and an inorganic lead halidespecies, PbX₂ (X═Cl, Br, I). The preparation methods for perovskitefilms using precursors can be categorized into three processes: 1)vacuum, 2) solution, and 3) hybrid. We invent none of these processes.And any of these processes known in the art can be used to complete ourinvention. Here, just to illustrate how the invention works, we choosevacuum deposition. Normally in this process the organic species areco-evaporated to form uniform planar perovskite films on the inorganiclayer which would be silicon, in this example, but could be germaniumetc. However, since in this invention the Sn (inorganic species in theperovskite) is already on the Si layer, only the organic species need tobe evaporated. So the Sn layer (on the Si film) is exposed to MAI vaporto form the tin perovskite (MASnI). While perovskite film deposition canbenefit from an electron separation layer, or transport layer, such asTiO₂, device scenarios without a scaffold such as TiO₂ or electrontransport layer (and hole transport layer) have achieved reasonableefficiencies. Moreover, silicon layers can form a tunnel junction forperovskite deposition. Here, instead of use of a wafer, we disclose amethod of forming a tunnel junction from the silicon thin-film layers.When Al is used as a catalyst with Si, any Al impurities in the Si serveas a p-type dopant in the Si film. A n-type layer can then be added toform the tunnel junction.

As already stated, in the invention disclosed herein, the crystallinesilicon thin-film can serve as the tunneling junction, or the metal fromthe eutectic alloy—in this case Sn—can be oxidized to form the thinoxide layer at the interface between the two component cells that servesas an efficient recombination layer. Alternatively, the metal film canremain un-oxidized. Ohmic contacts to semiconductors are typicallyconstructed by depositing thin metal films of a carefully chosencomposition.

A second challenge to commercialization, with regard to tandem designs,has to do with matching bandgaps. The bandgap of the top layer in thetandem configuration has to match the bandgap of the bottom layer,meaning that the bandgaps are not the same but complement each other andare synergistic in such a way as to maximize the power conversionefficiency (PCE) of the solar cell. For example, an ideal bandgap in thetop perovskite layer is between 1.65-1.7 eV for a 1.12 eV silicon filmbottom layer. Perovskite bandgaps are tunable, and one example of how totune them is to introduce Br (bromine). Doing this tunes the perovskitelayer to higher energies. In the present invention such tuning isachieved by adjusting the metal-semiconductor ratio and choosing theappropriate metal and semiconductor combination. Alternatively, suchtuning is achieved by the impurities picked up by the metal as itdiffuses to the surface of the silicon or germanium film, as in theprevious challenge.

Finally, a third challenge is to create a hysteresis free design. In thepresent invention, hysteresis is eliminated by using the same metal inthe electron extraction layer (EEL), or electron selector layer (ESL),as in the perovskite. For example, the EEL could be SnO₂ (tin oxide)when the perovskite is a tin perovskite. Tin oxide is known to obtain analmost hysteresis-free PCE.

When making actual solar cells and panels using perovskite layers intandem with silicon or germanium thin-films, it is possible to create abi-facial design, where light enters through both sides of the solarcell. In order to achieve this, very thin-films are used to insuretransparency on both ends, and light transmission through the layers.Such film thicknesses can be in the nanometers, ranging from 5-10 nm,10-20 nm, 20-30 nm, 30-40 nm, and so forth up to 1 micron. Transparentconducing oxides, glass, back contacts are used in this bi-facialdesign.

Normally, when designing the architecture of a monolithic heterojunctionperovskite silicon tandem solar cell either a tunnel junction or arecombination layer is deposited in order to electrically connect thetop perovskite cell to the bottom silicon cell. In one distinctiveembodiment of the invention disclosed here, the silicon film serves as amesoporous scaffold like TiO₂ and a polymer (P3HT) film underneath thesilicon film serves as a conducting layer following the technologyinvented by A. Chaudhari (U.S. Pat. No. 9,349,995 B2). The polymer filmhere also serves as a third semiconductor material for a triple junctionsolar cell for even higher efficiency.

In one embodiment of the invention, instead of using glass as asubstrate, organic materials such as polyimide can be used for flexible,roll-to-roll processing. Likewise, metal tapes with texture which areflexible can be used, following processes known in the art.

The following are examples of embodiments of the inventions recitedabove:

Example 1

The Sn segregates on the Si film as per process by P. Chaudharidisclosed in U.S. Pat. No. 9,054,249. Following perovskite filmdeposition processes known in the art, a dual source thermal evaporationsystem (Kurt J. Lesker, Mini Spectros) for depositing the perovskiteabsorbers is used to deposit the organic and inorganic components of theperovskite onto the Sn on Si. Ceramic crucibles are used in a nitrogenfilled glovebox. One source deposits the organic and one source depositsthe inorganic. For example, in the case of CH3NH3PbI3-xClx (mixed halideperovskite) which is proven to be an effective semiconductor absorberlayer in solar cells, the organic source is methylammonium iodide andthe inorganic source is PbCl2. In this invention, the methylammoniumiodide is deposited from the organic source, and the Cl2 is depositedfrom the inorganic source, and both onto the Sn layer on the Si film (orSn layer on the textured insulator if that is preferable). The molarratios of these chemicals needs to be determined through experimentationas do the thicknesses of the silicon and perovskite layers (thicknessdetermines current and absorption capabilities), but it is estimatedthat the silicon layer thickness would be between 20-80 μm while theperovskite layer would be under e.g. much thinner. As the substrate hasbeen heated, annealing of the perovskite substrate is not necessarycontrary to common practice and is a distinguishing feature of thisinvention. The films made using this process will be smooth and uniformsince the metal (Sn) is a layer in advance of the addition of the otherelements. Smoothness and uniformity are important for deviceperformance.

Example 2

Just like Example 1, but the Sn is deposited directly on the texturedbuffer layer—say MgO [111]. The Sn then spreads uniformly over thesubstrate. The other components of the perovskite film are added. Inaddition to forming a uniform, homogeneous film, as the components areadded the perovskite film crystallizes and replicates the [111] texturewhich improves the connection between the two semiconductors.

Example 3

Following the technology invented by A. Chaudhari (U.S. Pat. No.9,349,995 B2), a polymer film such as P3HT is deposited on the texturedbuffer layer—say MgO [111]—thereby obtaining texture itself. This layeris conducting. A silicon or other inorganic film is then deposited outof a Sn—Si eutectic melt onto the polymer film at low temperature (below400° C.) and as in the previous examples the Sn diffuses to the surfaceof the Si film, forming a very thin, uniform layer, which can now beused as the metal in the perovskite film as in the previous examples.The difference here is that the Si film serves as a scaffold(mesoporous) like TiO₂ and a polymer film underneath is a conductinglayer while also serving as a third semiconductor material for a triplejunction solar cell for even higher efficiency.

Example 4

Just like example 1, but the substrate used is organic, such aspolyimide, and is flexible and is thus capable of roll-to-roll (R2R)manufacturing.

Example 5: Stability

Following the patented procedure disclosed in U.S. Pat. No. 9,054,249 B2(incorporated by reference to its entirety herein) silicon or germaniumfrom a metal-inorganic eutectic alloy is deposited on glass. By choosinga non-toxic metal that can be used in a perovskite, such as Sn (tin) forexample, the Sn in the Sn—Si or Sn—Ge layer which is deposited on theglass can serve as the source of the metal to the perovskite bycontributing the Sn to the perovskite which is formed on top of theSi—Sn layer after the Si—Sn has been deposited. This can be achievedbecause the Sn generally segregates to the surface of the Si film. Thus,when the other chemical elements of the perovskites are deposited on theSn—Si film they combine with the Sn to form a tin perovskite layer. Thisprocess not only serves as a way of forming the perovskite film, butalso helps to eliminate the step of removing the Sn from the Si or Gefilm which is necessary for forming the Si thin-film on glass capable offunctioning as a tandem solar cell. Most importantly, the Sn in thisprocess will have some Si impurities or “dopant” which then serves tostabilize the Sn in the perovskite (particularly, the instability ofSn2+ ion in the presence of 02 and moisture is stabilized. Under ambientconditions, the Sn2+ ion will rapidly oxidize to its more stable Sn4+analogue. This process will destroy the charge neutrality of theperovskite structure and cause it to break down, resulting in theformation of oxides/hyrdoxides of Sn and MAI.

A good high vacuum system with two electron beam guns, is used todeposit tin and silicon independently. A glass substrate coated withtextured MgO is held at temperatures between 575 and 600° C. These arenominal temperatures. It is understood to one skilled in the art thatlower or higher temperatures can also be used depending upon thesoftening temperature of the glass substrate or the reaction kinetics ofeither tin or silicon with the MgO layers when used a substrates. A thintin film of approximately 10 nm thickness is deposited first. This isfollowed by a silicon film deposited at a rate of 2 nm per minute on topof the tin film. The silicon film nucleates heterogeneously on the MgOsurface to form the desired thin film. The film can now be cooled toroom temperature, where the film now comprises of two phases: tin and arelatively large grained and highly textured film of silicon on MgO. Thetin diffuses to the surface of the silicon film, driven by its lowersurface energy relative to the silicon surface. Rather than etching thefilm in a solution, which removes the Sn from the two phases, tin andsilicon, leaving behind a silicon film the Sn in the Si—Sn film can nowbe used as a surface on which to deposit the perovskite elementscombining to form a tin perovskite. Since the Sn has some impuritiesfrom the Si (as a result of the eutectic alloy), its stability isimproved and the tin perovskite film as a whole becomes stable. The Siimpurities serve as dopant. Such impurities can alternatively beintroduced by doping procedures known in the art.

For the Si—Sn deposition we have used two electron beam guns as anillustrative example. It is understood to one skilled in the art thatother methods such as a single gun with multiple hearths, chemical vapordeposition, thermal heating, or sputtering can also be used, forexample. For the perovskite elements, such as methylammonium halides,deposition can be by low-temperature solution methods (such asspin-coating, but also others). Since low-temperature (below 100° C.)solution-processed films tend to have considerably smaller diffusionlengths, a higher temperature method may however be favorable so long asit does not melt the underlying substrate.

Example 6: Bandgap Tuning

Following the patented procedure disclosed in U.S. Pat. No. 9,054,249 B2silicon or germanium from a metal-inorganic eutectic alloy is depositedon glass. By choosing a non-toxic metal that can be used in aperovskite, such as Sn (tin) for example, the Sn in the Sn—Si or Sn—Gelayer which is deposited on the glass can serve as the source of themetal to the perovskite by contributing the Sn to the perovskite whichis formed as a layer on top of the Si layer after the Si—Sn has beendeposited and the Sn segregates on the surface of the Si film. Thus,when other chemical elements of the perovskites are deposited on the Snon Si film they can combine with the Sn to form a tin perovskite layer.Meanwhile, the Sn layer remains to serve as an intermediaterecombination layer electrically connecting the inorganic sub-cell tothe perovskite cell on top. This process not only serves as a way offorming the perovskite film, but also eliminates the step of removingthe Sn from the top of the Si or Ge film which would otherwise benecessary for forming the Si thin-film on glass capable of functioningas a tandem solar cell. Importantly, for the purposes of thisdisclosure, the Sn (or other metal) in this process will affect thebandgap of the Si film and Sn perovskite film because of the impuritiesin the Si (in or outside of the Si crystal lattice) and the impuritiesin the Sn (in or outside of the Sn crystal lattice). By changing theratio of the Sn to Si (eutectic alloy), and/or by changing the metal orsemiconductor (for example using Au and/or Ge), the bandgap can bechanged to a number ideal in compatibility with the overlying perovskitefilm. The invention applies to any metal-inorganic film or eutecticalloy, not just Sn—Si, and any perovskite film on top, Sn or otherwise.

A good high vacuum system with two electron beam guns, is used todeposit tin and silicon independently. A glass substrate coated withtextured MgO is held at temperatures between 575 and 600° C. These arenominal temperatures. It is understood to one skilled in the art thatlower or higher temperatures can also be used depending upon thesoftening temperature of the glass substrate or the reaction kinetics ofeither tin or silicon with the MgO layers when used a substrates. A thintin film of approximately 10 nm thickness is deposited first. This isfollowed by a silicon film deposited at a rate of 2 nm per minute on topof the tin film. The silicon film nucleates heterogeneously on the MgOsurface to form the desired thin film. The film can now be cooled toroom temperature, where the film now comprises of two phases: tin and arelatively large grained and highly textured film of silicon on MgO. Thetin diffuses to the surface of the silicon film, driven by its lowersurface energy relative to the silicon surface. The Sn can now be usedas the Sn in the tin perovskite layer on top and can thereby tune thebandgap of the perovskite layer due to Si impurities in the Sn.Additionally, the Sn layer on the Si serves as an intermediaterecombination layer electrically connecting the inorganic layer to theperovskite layer. Finally, the Si layer is affected by the Snimpurities, such that it can be tuned to an ideal bandgap number forsynergy with the perovskite film.

For the Si—Sn deposition we have used two electron beam guns as anillustrative example. It is understood to one skilled in the art thatother methods such as a single gun with multiple hearths, chemical vapordeposition, thermal heating, or sputtering can also be used, forexample. For the perovskite elements, such as methylammonium halides,deposition can be by low-temperature solution methods (such asspin-coating, but also others). Since low-temperature (below 100° C.)solution-processed films tend to have considerably smaller diffusionlengths, a higher temperature method may however be favorable so long asit does not melt the underlying substrate.

Example 7: Hysteresis-Free

Following the patented procedure disclosed in U.S. Pat. No. 9,054,249 B2silicon or germanium or another semiconductor from a metal-inorganiceutectic alloy is deposited on glass. By choosing a non-toxic metal thatcan be used in a perovskite, such as Sn (tin) for example, the Sn in theSn—Si or Sn—Ge layer which is deposited on the glass can serve as thesource of the charge carrying film required for perovskite solar cellperformance, by simply oxidizing the metal which is formed as a layer ontop of the Si layer after the Si—Sn has been deposited and the Snsegregates on the surface of the Si film, to form, say, SnO2. The use ofSn as the charge carrying film or EEL (electron extracting layer), orESL (electron selective layer), or intermediate recombination layer,eliminates hysteresis due to the correspondence of the Sn in the Sn02with the Sn in the tin perovskite layer (MASnI).

A good high vacuum system with two electron beam guns, is used todeposit tin (or another metal) and silicon (or another semiconductor)independently. A glass substrate coated with textured MgO (or anotheroxide) is held at temperatures between 575 and 600° C. These are nominaltemperatures. It is understood to one skilled in the art that lower orhigher temperatures can also be used depending upon the softeningtemperature of the glass substrate or the reaction kinetics of eithertin or silicon with the MgO layers when used a substrates. A thin tinfilm of approximately 10 nm thickness is deposited first. This isfollowed by a silicon film deposited at a rate of 2 nm per minute on topof the tin film. The silicon film nucleates heterogeneously on the MgOsurface to form the desired thin film. The film can now be cooled toroom temperature, where the film now comprises of two phases: tin and arelatively large grained and highly textured film of silicon on MgO. Thetin diffuses to the surface of the silicon film, driven by its lowersurface energy relative to the silicon surface. By oxidizing the tinlayer on the silicon film surface to form a metal oxide, the necessaryelectron extracting layer (EEL) transportation layer can be formed onwhich to deposit the perovskite film. Using Sn instead of the commonlyused TiO2 avoids the necessity of sintering the metal in order tocrystallize the film since the metal has been deposited on a heatedsubstrate at eutectic temperature such that the Sn will crystallize.Moreover, since the eutectic temperature of the Si—Sn system is muchlower than 500° C., substrates such as glass or others that require lowtemperature can be used, not to mention the cost savings in powerconsumption associated with lower temperature processing. The bandgap ofthe perovskite layer is adjusted to match the bandgap of the siliconfilm (1.11 eV) to maximize the efficiency.

For the Si—Sn deposition we have used two electron beam guns as anillustrative example. It is understood to one skilled in the art thatother methods such as a single gun with multiple hearths, chemical vapordeposition, thermal heating, or sputtering can also be used, forexample. For the perovskite elements, such as methylammonium halides,deposition can be by low-temperature solution methods (such asspin-coating, but also others). Since low-temperature (below 100° C.)solution-processed films tend to have considerably smaller diffusionlengths, a higher temperature method may however be favorable so long asit does not melt the underlying substrate.

In the present invention, the terms ‘textured’ and ‘large grain’ havethe following meaning: ‘textured’ means that the crystals in the filmhave preferential orientation either out-of-plane or in-plane or both.For example, in the present invention the films could be highly orientedout-of-plane, along the c-axis. By ‘large grained’ it is meant that thegrain size is greater than or equal to the film thickness.

While the present invention has been described in conjunction withspecific embodiments, those of normal skill in the art will appreciatethe modifications and variations can be made without departing from thescope and the spirit of the present invention. Such modifications andvariations are envisioned to be within the scope of the appended claims.

What is claimed is:
 1. A method of making a non-toxicperovskite/inorganic thin-film tandem solar cell comprising the stepsof: depositing a textured oxide buffer layer on a glass substrate;depositing a metal-inorganic film from a eutectic alloy on said bufferlayer; and depositing perovskite elements on said metal-inorganic film,forming a perovskite layer based on said metal from said metal-inorganicfilm, incorporating said metal into said perovskite layer, wherein saidperovskite layer is stable, hysteresis-free, and has a bandgap thatmatches the bandgap of the metal-inorganic.
 2. The method of claim 1,wherein said perovskite layer is made stable by impurities or doping. 3.The method of claim 2, wherein said correspondence between said metalfrom said metal inorganic film and said metal forming the perovskitelayer eliminates hysteresis.
 4. The method of claim 1, wherein thebandgap of the perovskite layer and the bandgap of said metal inorganicfilm is made to match by impurities or doping.
 5. A bi-facial tandemsolar cell assembly comprising; a glass substrate; a transparent oxidebuffer layer; an inorganic film; an EEL layer; a perovskite film; atransport hole transport layer; and a transparent conducting oxide for aback contact.
 6. The assembly of claim 5, wherein a top side and abottom side of the solar cell are transparent.
 7. The assembly of claim5, wherein each layer of the tandem solar cell are thin enough to betransparent.